1. Field of the Invention
The present invention relates to a motor control device for an electric injection molding machine.
2. Description of the Prior Art
Injection molding machines are known to have an injection unit which operates fully hydraulically or by a hydraulic-electric driving system. Nowadays, injection units have become available which are driven fully electrically. Examples of such electric injection units are shown, for example, in Japanese Patent Unexamined Publication Nos. 61-24423 and 61-31221.
In the electric injection unit disclosed in Japanese Patent Unexamined Publication No. 61-24423, the resin pressure in the injection cylinder is controlled on the basis of the pressure value which is calculated from electric current and voltage of the electrical power supplied to a driving motor. The calculated resin pressure, however, largely differs from the actual resin pressure because the driving power calculated from the current and voltage does not correctly reflect the actual pressure due to an energy loss in the transmission system including a nut and a screw shaft between the electric motor and the plasticizing screw.
The difference between the calculated resin pressure and the actual resin pressure is increased particularly when the screw is being accelerated or decelerated because the current and the voltage have been changed due to inertia of the motor and the transmission system.
The injection unit disclosed in Japanese Patent Unexamined Publication No. 61-31221 is designed to directly detect the resin pressure, so that no substantial influence is caused by acceleration and deceleration of the screw. However, a considerably large error is caused also in this case due to presence of the nut and screw shaft between the pressure detector and the plasticizing screw.